A combustion exhaust gas discharged from a coal-fired furnace or a coal-fired thermal power plant contains sulfur oxide (SOx), and a desulfurization apparatus is installed in order to treat sulfur oxide (SOx). As a method of removing sulfur oxide from a gas containing sulfur oxide in the desulfurization apparatus, there is given a method involving allowing the gas containing sulfur oxide to react with an alkaline agent and oxygen in an absorbing liquid. Waste water generated in such method of removing sulfur oxide contains a nitrogen compound and a chemical oxygen demand (COD) component, and hence waste water treatment is performed for their removal. However, there is a problem of degradation in performance of a waste water treatment apparatus for the waste water treatment.
As a technology for solving the above-mentioned problem, in Patent Literature 1, there is disclosed “a wet-type flue-gas desulfurization method, including: using a soot-mixing wet-type flue-gas desulfurization apparatus in order to remove sulfur oxide in an exhaust gas; and performing first gas-liquid contact and then second gas-liquid contact in a series in regions adjacent to each other, the first gas-liquid contact including spraying a first alkaline agent-containing liquid to an exhaust gas to allow gas-liquid contact therebetween, the second gas-liquid contact including allowing gas-liquid contact between the exhaust gas after the first gas-liquid contact and a second alkaline agent-containing liquid containing an absorber to remove mainly sulfur oxide in the exhaust gas in the presence of an oxygen-containing gas for oxidation of sulfur oxide, the wet-type flue-gas desulfurization method including: extracting a slurry containing solid matter generated through the second gas-liquid contact to use at least part thereof as the first alkaline agent-containing liquid; extracting the first alkaline agent-containing liquid spontaneously separated from the exhaust gas after the first gas-liquid contact through precipitation separation; and feeding the extracted first alkaline agent-containing liquid to a waste water treatment apparatus subsequent to the soot-mixing wet-type flue-gas desulfurization apparatus.” According to the method disclosed in Patent Literature 1, the concentration of an oxidizing substance, such as a peroxide, in the waste water can be reduced. As a result, degradation in performance of the waste water treatment apparatus can be suppressed.